High frequency capacitors are used in a variety of applications, ranging from industrial lasers to implantable medical devices, such as heart defibrillators. Implantable heart defibrillator pulse generators require approximately 30 joules of energy to start the human heart and improved energy storage dielectrics will allow for the further miniaturization of these devices, which are currently the size of a pocket watch. Due to their fast discharge (less than one second), capacitors typically supply high power densities and small energy storage densities compared to batteries or supercapacitors.
Unlike batteries that store energy via a chemical mechanism, capacitors store energy in an electrostatic field that induces positive and negative charges on the plates of the device. The maximum energy that can be stored by a dielectric is determined by the relative permittivity and breakdown strength of a material. Energy storage density for a linear dielectric is calculated using equation 1:
                    J        =                                            ∫                              P                o                                            P                max                                      ⁢                          E              ⁢                              ⅆ                P                                              ≈                                                    ɛ                o                            ⁢                              ɛ                r                            ⁢                              E                2                                      2                                              (        1        )            in which J is the energy density, E is the electric field sustained by the dielectric, P is the induced polarization of the dielectric, Pmax is the maximum induced polarization, Po is the polarization at zero electric field, ∈r is the relative permittivity of the material, and ∈o is the permittivity of free space. Thus, an ideal material for dielectric energy storage would possess a high relative permittivity, high breakdown strength, and a low loss tangent under high applied electric fields. It is appreciated that due to the squared dependence of energy storage density on applied electric field, it is advantageous to maximize the breakdown strength of these materials.
FIG. 1 shows the energy storage density as a function of maximum processing temperature for several heretofore known materials. FIG. 1 includes both lead-containing and lead-free oxide thin films, as well as high energy density polymers, composite materials, and several glasses. To achieve a high energy storage density, these materials often exhibit a compromise between the relative permittivity and breakdown strength. For example, an energy density in excess of 35 J/cm3 using an alkali-free glass with a relative permittivity of 6 and a breakdown strength of 12 MV/cm is shown, as well as an energy density of 22 J/cm3 for lead lanthanum zirconium titanate (PLZT) films with a relative permittivity of 1100 and a breakdown strength of 1.6 MV/cm. By demonstrating an electrode with an increased breakdown strength of 4.3 MV/cm, PLZT films on nickel foils achieved a best-case energy density of 85 J/cm3, the maximum value reported in the literature. The loss tangent for these PLZT films was between 0.05 and 0.08.
FIG. 2 shows the breakdown strength as a function of relative permittivity for several materials reported to have a high energy storage density. As seen in FIG. 2, many materials fall above the historical “best-fit” line, primarily due to increases in the maximum achievable breakdown strength of the material.
Historically, lead-containing materials have demonstrated superior properties, such as a larger dielectric constant, piezoelectric coefficient, and energy storage density. However, human health and environmental concerns surrounding lead-based materials have led researchers to target lead-free materials for applications ranging from energy storage to piezoelectric devices. Therefore, an improved lead-free dielectric material for use in capacitors would be desirable.